Organic semiconductors (OSCs) are expected to revolutionise the manufacturing process of the thin film field-effect transistors (TFTs) used for display technologies. Compared with the classical Si based field-effect transistor (FETs), organic TFTs can be fabricated much more cost-effectively by solution coating methods such as spin-coating, drop casting, dip-coating, and more efficiently, ink-jet printing. Solution processing of OSCs requires the molecular materials to be 1) soluble enough in non-toxic solvents; 2) stable in the solution state; 3) easy to crystallise when solvents are evaporated; and most importantly, 4) to provide high charge carrier mobilities with low off currents. In this context, pentacenes and hetero-acenes with solublising substituents, have shown to be promising classes of p-type OSC materials. Notably, unsymmetrically substituted pentacenes have shown hole mobility greater than 3 cm2/Vs, as disclosed in WO 2009/155106 A1 while fluorinated anthracenodithiophene derivatives (F-ADTs) have shown hole mobility greater than 1 cm2/Vs, as disclosed in US 2008/0128680 A1 and Payne et al., J. Am. Chem. Soc., 2005, 127 (14), 4986; and Subramanian et al., J. Am. Chem. Soc. 2008, 130(9), 2706-2707.
However, some major drawbacks remain for these materials, which include low photo and environment stability, particularly in solution states, and a low temperature of phase transition and melting point. Also for future OLED backplane applications, which demand higher source and drain current, the mobility and processibility of currently available materials needs further improvement.
Acenes larger than pentacene keep attracting interests in the quest for novel OSCs due to the predicted lower reorganization energy (see Deng et al., J. Phys. Chem. B, 2004, 108, 8614) and the potential higher charge carrier mobility (see Cheng et al., J. Chem. Phys., 2003, 118, 3764). However, linear elongation of the aromatic cores by fusing additional benzene rings is witnessed by the decreased stability and solubility in organic solvents, which compromised the practical application of these analogues as OSC materials (see Purushothaman et al., Org. Lett., 2010, 12(9), 2060). Interestingly, polycyclic aromatic hydrocarbons much larger than pentacene have either been synthesized as nano materials (see Yang et al., J. Am. Chem. Soc., 2008, 130 (13), 4216) or existed in nature as dye stuffs without stability issues due to their 2-D fusing features. This type of structure is most notably represented by free-standing graphene, a class of intrinsic 2-D polycyclic aromatic system, of which large charge carries mobilities exceeding 104 cm2/Vs have been observed under ambient conditions (see Geim et al., Nat. Mater., 2007, 6(3), 183-191; and Allen et al., Chem. Rev., 2010, 110(1), 132-145), and exceeding 2×105 cm2/Vs have been achieved under optimised conditions (see Bolotin et al., Solid State Commun., 2008, 146, 351).
Therefore, there is still a great need for new OSC materials that show good electronic properties, especially high charge carrier mobility, good processibilty and high thermal and environmental stability, and especially a high solubility in organic solvents.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, and do especially show good electronic properties, especially high charge carrier mobility, good processibilty and high thermal and environmental stability, and especially a high solubility in organic solvents. Another aim of the invention was to extend the pool of organic semiconducting materials available to the expert.
It was found that these aims can be achieved by providing compounds as claimed in the present invention. These compounds are based on flavanthrone, a industrially available dye stuff with a fused polycyclic aromatic ring structure as shown below, as the starting material.

For the purposes of the present invention flavanthrone has been made soluble in organic solvents, so that it becomes solution processible, by the introduction of solublising functional groups through aromatisation of its quinoid structure into a new core unit, i.e. benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine (hereinafter also shortly referred to as “flavanthrene”). The inventors of the present invention have found that these flavanthrene derivatives exhibit high solubility in organic solvents, especially those that are typically used in organic electronic device manufacture, and in addition show good thermal stability and high charge carrier mobilities.
No examples of flavanthrene based materials have been reported up to date in the literature.